There are many situations in which mechanical vibrations need to be accurately transduced to electrical signals. Various techniques have been used for detecting vibrations in a single direction (e.g., piezo electric, strain gauge, etc). Such systems usually constrain the moving body in the plane of two orthogonal coordinates but allow it to move freely in the third orthogonal direction. Thus, these systems detect vibrations in one direction.
There are other situations where vibrations in a plane need to be transduced, in which case the moving body needs to be fixed in one direction but allowed to vibrate freely in the other two directions. This is the case for a vibration transducer for a guitar string or a stringed musical instrument.
It is well known that a guitar string when picked (i.e., plucked) initially vibrates in the direction of the picking action, but it then gradually precesses away from that direction into a direction 90.degree. away and eventually back to the original direction. This precession around an ellipse is described in Rayleigh's 1877 classic, "The Theory of Sound", Dover 1945.
Various electro-magnetic sensors have been built for guitar strings (see U.S. Pat. Nos. 4,378,722--Isakson and 3,571,483--Davidson). But these do not transduce high frequency signals well because of the inductance of the coil pickups.
Nourney in U.S. Pat. Nos. 4,228,715 and 4,292,875 discloses using a multiplicity of strain gauges to transduce string vibrations. The strain gauges also transduce the longitudinal vibrations of the string which occur at double the fundamental frequency of vibration. It is believed that such arrangements are not practicable because strain gauges are very difficult to mount reliably and because of the need for temperature compensation circuitry. Moreover, a transducer for use in accurately controlling a remote synthesizer should be insensitive to the double frequency caused by the longitudinal vibrations.
Another U.S. Pat. No. 3,453,920 (Scherer) discloses the use of one or two piezo-electric elements arranged under the string support. If the two piezo-electric elements are arranged with opposing polarities they become, in theory, insensitive to vertical motion while transducing horizontal motion. We found this not to be true due to mismatching of the piezo-electric elements and also because of difficulties in mounting the miniature piezos in such a way as to prevent shear stresses, which in turn generate extraneous voltages.